docs: def export_certificate(ctx, slot, format, certificate):
      """
      Export a X.509 certificate.
  
      Reads a certificate from one of the slots on the YubiKey.
  
      \b
      SLOT        PIV slot to read certificate from.
      CERTIFICATE File to write certificate to. Use '-' to use stdout.
      """
      controller = ctx.obj['controller']
      try:
          cert = controller.read_certificate(slot)
      except APDUError as e:
          if e.sw == SW.NOT_FOUND:
              ctx.fail('No certificate found.')
          else:
              logger.error('Failed to read certificate from slot %s', slot,
                           exc_info=e)
      certificate.write(cert.public_bytes(encoding=format))